This invention relates to a process for the boiling extrusion of products, preferably for human or animal consumption, in which a predetermined amount of solid is pre-conditioned, before the boiling extrusion, by means of a predetermined amount of live steam and a predetermined amount of water in a conditioning means, in order to moisten and preheat the product, and in which a predetermined amount of water is fed into an extruder following the conditioning means and an adjustable steam pressure is thereby produced in the extruder in order to influence the degree of expansion of the extrudate, wherein regulation of this steam pressure is effected by a predetermined amount of steam which is let out, locally and quantitatively, at the extruder.
The installation for carrying out the process comprises an extruder for said boiling extrusion, a conditioning means (also termed a pre-conditioning means) for the moistening and preheating of the product introduced into the extruder, a product transfer line which connects the conditioning means to the extruder, a metering means for the metered supply of a crude product into the conditioning means, and a live steam supply and a water supply into the conditioning means and into the extruder in each case, as well as a steam outlet at a predetermined point on the extruder.
During the boiling extrusion of animal foodstuffs, particularly fish food and pet foods, and in the modification of cereals, the product is advantageously extruded at a relatively high water content between 20-35% and at temperatures of 100-180.degree. Celsius. It is primarily the parameters of temperature, water content, dwell time and shear (in the extruder) which are decisive for the good conglutination of starch.
In order to prolong the "time" parameter depending on the temperature and moisture content of the product to be processed, a conditioning stage (which is also termed a pre-conditioning stage) is advantageously used before the extrusion stage. Moistening (also termed dampening) and preheating of the product which is subsequently to be extruded occurs in a conditioning stage such as this. At the same time, this conditioning operation has to be effected so that the conditioned product is not yet plasticised.
After the conditioning stage, the boiling process is effected in the extruder or is continued therein in that the temperatures can be increased still further and plasticisation occurs.
Under the optimum extrusion conditions, this plastic mass is inflated very considerably at temperatures above 100.degree. Celsius by the steam pressure associated therewith, so that it then solidifies to form a foam-like structure similar to bread when it emerges from the die at the end of the extrusion process, during the emergence of moisture and the expansion of the product which are associated therewith, and during the cooling due to expansion which is associated therewith.
The degree of expansion is determined by the boiling process. In order to separate the mutual influencing of boiling and shaping, a steam outlet is installed at a predetermined point in the extruder, through the opening of which outlet the steam which is present in the product can escape at a predetermined pressure which corresponds to the desired product. Ultimately, the temperature of the product falls downstream of the shaping die, due to the evaporation of water, i.e. it falls towards 100.degree. Celsius at atmospheric pressure.
The degree of expansion of the extrudate can easily be influenced by regulating the steam pressure at this steam outlet. The water which evaporates during expansion, and which can amount to up to 7% of the extruder throughput, has a very large energy content.